Add font glyph fallback mechanism to the tiles build

[Qrox]

Summary

SUMMARY: Interface "Fallback fonts can now be specified for the tiles build"

Purpose of change

The font currently shipped with the game (unifont) has the merit of wide unicode support, but its pixelated glyphs doesn't actually look pretty. Other fonts with prettier glyphs, however, often support only a very limited subset of unicode glyphs. This PR implements font fallback mechanism for the tiles build, so the user can specify the main font to display the most common glyphs, and fallback fonts to display rare glyphs correctly when the main font doesn't provide these glyphs.

Describe the solution

The "typeface" value in config/fonts.json can now be an array specifying multiple typefaces. A new child class of Font, FontFallbackList, is added, which loads the specified typefaces and checks if a given glyph is provided with the typefaces. The first typeface from the array that provides the glyph is used to render the glyph.

Testing

Changed the typeface list and started the game, observed that glyphs not provided with the main font are correctly rendered with the glyphs from the fallback font.

Additional context

This is only for the tiles build. For the curses build, font fallback probably can only be achieved by configuring the terminal software.

In the above screen shot, the Latin/Cyrillic characters are rendered with UbuntuMono (bold), and the CJK characters are rendered with unifont as the fallback font.